Energy Recovery Ventilation

Proper ventilation is essential for maintaining good indoor-air quality, yet it places an additional burden on heating and cooling equipment, which must condition air that will soon be exhausted from the building. Energy recovery ventilation (ERV) systems capture thermal energy and moisture from the exhaust (inside to outside) airstream and transfer it to the intake (outside to inside) airstream, saving energy and potentially improving humidity control.

The first step to deploying ERV is to perform an energy analysis on the building. If you can’t afford a full energy analysis, look into demand-controlled ventilation (DCV) as an alternative. As a general rule, if the building’s occupation fluctuates, choose DCV; if occupancy is mostly constant, invest in ERV.

Although the savings from ERVs vary depending on climate, geographic locations with hot, humid summers are particularly well suited to these systems. Other benefits of ERV systems include:

Reduced HVAC energy consumption. ERV systems save energy by reducing the need to cool or heat outside air. Although energy consumption from fans can rise after an ERV system installation, the savings in heating and cooling energy generally far outweigh any increases.

Reduced peak demand. HVAC systems are some of the biggest contributors to peak demand—electricity use during the times of day when the utility is required to produce the most energy and when rates are highest. By reducing the amount of air that the HVAC must heat or cool, an ERV system can help lower a building’s peak demand, thereby lowering its electric bill even further.

Improved humidity control. ASHRAE and others have documented a growing discrepancy between building humidity load and air-conditioner sensible heat ratio (the HVAC unit’s ability to dehumidify the air). Improved building energy efficiency has decreased sensible cooling loads (for example, heat produced by inefficient lighting and plug loads), but latent loads—including occupant respiration and moist ventilation air—have remained essentially the same. Unfortunately, the HVAC industry still hasn’t entirely kept up. Today’s HVAC units are required to remove more latent heat than they were designed for, leading to higher indoor humidity levels. By predrying the incoming ventilation air, ERV systems can mitigate these conditions.

Appropriate ventilation. In some cases, HVAC systems may not be bringing in enough outside air to provide proper ventilation. Because an ERV system reduces the energy needed to condition outside air, you can increase ventilation air intake, thereby improving indoor-air quality. ERV systems also help you meet updated building codes without increasing energy consumption.

What are the options?

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ERV systems can transfer two types of heat: sensible and latent. “Sensible heat” refers to the heat content of the air itself, and it is measured with a standard thermometer. Latent heat is the amount of energy required to first evaporate water into vapor (humidity) and then remove that water from the water vapor via condensation. Measure latent heat with a wet-bulb thermometer.

Ventilation systems that transfer both sensible and latent heat (a process called total enthalpy) are referred to as ERV devices; those that only transfer sensible heat are called heat recovery ventilation (HRV) devices. The main difference between the two is in their treatment of moisture in the air. ERVs can modify humidity levels; HRVs can’t. For this reason, HRV systems may be desirable in tightly sealed spaces that are operating within humid climates, especially if the space has continuous vapor barriers that cause moisture from indoor activities to become trapped. Because an HRV can’t transfer moisture from the exhausted air to the incoming fresh air, the result is dehumidification in the conditioned space. ERVs, by contrast, need to ventilate more to reduce humidity levels. Typically, HRV systems do not meet the 50% energy recovery effectiveness level required by the International Energy Conservation Code of 2015.

Enthalpy wheels. Most often employed in the cooling season in humid climates, an enthalpy wheel is made up of heat- and moisture-adsorbing material (such as a desiccant) that rotates between the incoming outside air and outgoing exhaust air ducts (Figure 1). As the enthalpy wheel rotates, it removes water vapor from the moist outside air and transfers it to the dry, conditioned exhaust air that is leaving the building. Simultaneously, the wheel precools the hot incoming outside air and transfers that heat to the cool, conditioned exhaust air.

Figure 1: Mechanics of an enthalpy wheel

Enthalpy wheels are ideal for saving energy because of their high heat-transfer effectiveness, but they can also increase the pressure drop in HVAC ductwork, which will require additional fan energy. The values in this image represent typical conditions for a hot, humid climate in the summer months.

The enthalpy wheel saves electrical energy by precooling and dehumidifying the intake air, thereby reducing the load on both the refrigerant compressor and the air-handler fan. When conditions allow for partial loads, you can either reduce the wheel speed or add a bypass duct to lessen the load on the fans even further.

Enthalpy wheels are among the most common types of ERV systems; they typically yield high effectiveness values—the efficiency with which heat is moved from one place to another. This technology can have total effectiveness values of 75% or more, but proper cleaning and maintenance (at least once a year) is essential to ensure that dirt and debris don’t build up on them, which can reduce heat-transfer efficiency and increase the pressure drop in the ductwork.

Fixed-plate heat exchangers. Unlike enthalpy wheels, fixed-plate heat exchangers don’t have any moving parts. Instead, they drive intake and exhaust air through an alternating series of separate, sealed parallel plates, effectively transferring heat between the two airstreams (Figure 2).

Figure 2: Configuration of a fixed-plate heat exchanger

As with enthalpy wheels, fixed-plate heat exchangers can employ a bypass duct under part-load conditions to reduce fan energy consumption. The values in this image represent typical conditions for a hot, humid climate in the summer months.

Fixed-plate heat exchangers are available in a number of different configurations (two common options are horizontally or vertically oriented plates) and can be made to transfer moisture as well as heat by using desiccant materials to separate the airstreams. However, many fixed-plate heat exchangers use materials that only result in sensible heat transfer, such as aluminum or plastic; as a result, they yield lower net savings and higher indoor humidity levels than the moisture-transferring versions.

The total energy effectiveness of fixed-plate heat exchangers varies based on factors such as size and configuration, but it’s possible to find units with effectiveness values that are comparable to those of enthalpy wheels. And because no moving parts are involved, fixed-plate heat exchangers can use less energy and have lower maintenance costs than enthalpy wheels.

It’s important to maintain a clean air supply when using a fixed-plate heat exchanger. In applications where lots of particulates (for example, dust or smoke) are present, the unit could become clogged and be difficult to repair, particularly if it’s hard to reach.

Heat pipes. When two air streams pass through the heat pipes—one on the supply side and the other on the exhaust side—the temperature difference between the two will heat or cool the refrigerant.The refrigerant then changes phasefrom a liquid to a vapor and back again, which transfers energy from one side to the other. Most of the time, heat pipes transfer sensible energy, but if the surrounding air is cooled below its dewpoint, condensation forms on the pipe, which results in some latent heat transfer (Figure 3). The sensible effectiveness of heat pipes is 45% to 65%.

Figure 3: Heat pipe system

Heat pipes can also be used as indirect evaporative coolers—automated sprayers can spray water on the exhaust side of the pipe to precool the supply air.

Heat pipes can be a great option because they require no energy to operate and have no moving parts. This causes them to last a long time, with simple periodic cleaning for maintenance. You can even order them with a protective coating to guard against corrosion.

Run-around loops. To cool process water, the system must reject heat; a run-around loop (also known as a “wrap-around loop”) uses that rejected heat to preheat incoming outside air for ventilation purposes (Figure 4). In warm, humid climates, they can also reheat cooled, dehumidified air by transferring heat from outside. This technology circulates fluid between a system’s airstreams or heat sources, allowing for energy transfer between process loads and ventilation air without requiring that those elements be physically together.

Figure 4: Run-around energy recovery loop with dehumidification

A run-around loop recovers energy from exhaust air by transferring heat provided by warm outside air. This method only works in warm, humid climates.

Rooftop ERVs. ERVs can also be installed on rooftops like other HVAC units. Vendors currently offer rooftop ERV units that house enthalpy wheels that can pivot out of the airstream while the rooftop unit is in economizer mode.

How to make the best choice

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ERV technology falls under the category of HVAC optimization; you need to assess your HVAC system before making changes to it.

Pick the right components

Before you begin, it’s important to be sure that you’re considering the right kind of system for your situation.

Determine the space required. The size of an existing system will affect which components you choose and how much they cost. Do this before you plan your upgrades.

Consider supply and exhaust locations. If your system’s supply and exhaust airstreams are next to each other, you have the option of installing enthalpy wheels and heat pipes. If the airstreams are separated, run-around loops are likely a better option.

Consider a run-around loop. If your facility isn’t in a warm, humid climate and doesn’t require dehumidification, a run-around loop can save energy and money.

Choose the right building

Some buildings are better candidates for ERV than others. Here are some characteristics to look for.

A moderate-to-extreme heating or cooling climate. Given that ERV can reduce the conditioning load from ventilation, buildings in climates where a lot of energy is required to heat or cool the outdoor air stand to benefit the most, whereas those in climates where little conditioning is required or where economizer operation is common will save less. Facilities with large refrigeration loads, such as supermarkets, can use ERV to reduce the humidity load that display cases would otherwise have to remove.

Heavy ventilation requirements. Buildings that require large amounts of outdoor air are likely to be good candidates for ERV because they will have correspondingly higher heating and cooling ventilation loads. For buildings that are only open for a few hours per day but that have high ventilation loads during those times, using timers to shut off ventilation fans during unoccupied hours is a lower-cost approach.

New construction. It’s easier to integrate ERV systems into new buildings, where you can optimize the ductwork from the beginning. Because ERV systems reduce the heating and cooling load on the HVAC system, much of that equipment can be downsized, yielding lower up-front costs and larger energy savings.

Determine cost-effectiveness

Costs and savings can vary from site to site based on factors like local climate; the size, type, capacity, and complexity of the ERV unit; and the HVAC system to be installed. Although there are some general rules for estimating installed costs and reductions in HVAC tonnage, they’re rarely accurate. The best way to assess these criteria is to use a simulation tool like the US Environmental Protection Agency’s (EPA’s) ERV Financial Assessment Software Tool (see the EPA’s School Advanced Ventilation Engineering Software website for more information and to get a free copy) or vendor-provided calculators such as those from Airxchange,RenewAire, and AAON.

ERV and HRV systems are well-established, and installed costs are relatively stable, but savings can vary depending on the region and the settings used. A study conducted at Purdue University showed favorable economics for ERV systems in a variety of buildings (Figure 5). Researchers investigated savings in four types of buildings across the country using a detailed simulation model. Simple payback periods for retrofit installations of enthalpy wheels ranged from two years to more than five years in hot and humid climates.

Figure 5: Annual HVAC energy cost savings from enthalpy wheels

Although demand reductions (A) and overall energy bill savings (B) will depend heavily on actual occupancy patterns, the relatively higher percentage of savings in restaurants and retail stores highlights these types of facilities as good candidates for ERV systems.

When Airxchange conducted a case study at the Boston House of Blues (PDF), its researchers found that enthalpy wheels became stuck after years of exposure to airborne particles, causing heating and cooling systems to work overtime. After selecting new enthalpy wheels that were easier to maintain, both the cooling and heating loads dropped roughly 60%.

Improperly set ERV systems can actually increase HVAC energy consumption. It’s important to maintain and commission your ERV system regularly—both to maximize savings and to ensure that those savings persist over the life of the unit. Maintenance typically involves inspecting and changing filters, as well as vacuuming the energy-exchange element. Enthalpy wheels tend to require the most maintenance, but they also offer the highest savings.

What’s on the horizon?

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ERVs and HRVs are commercially available; both are more common in colder climates. According to the US Department of Energy, it’s possible to retrofit air handlers with energy recovery systems, which will reduce installation costs and complexity for existing buildings. Vendors are already pursuing these opportunities in the market. Additionally, there is also opportunity to create better simulation models that predict energy savings and costs across geographical climates.